Directional radio beacon



Um 1%. CARL-ERIK GRANQVIST 2,13%,23

DIRECTIONAL RADIO BEACON Filed Oct. 29, 1940 s Sheets-Sheet 1 CODE I'IECHHNISN TRANSMITTER BY m n w 29, WW.

CARL -ERIK GRANQVIST DIRECTIONAL RADIO BEACO N Filed 001'.- 29, 1940 3 Sheets-Sheet 2 CODE MECHANISM Fig. 51

TRANS/7 I TTE I? TRANS M/TTER CARL-ERIK GRANQWST 23%,623

DIRECTIONAL RADIO BEACON Filed Oct. 29, 1940 s Sheets-Sheet 5 CONTROL, om/um'oi'? MMfLNVENTQR BY Patented Dec. 29, I42

Application October 29, 1940, Serial No. 363,280 In Sweden November 28, 1939 9 Claims.

My invention relates to directional radio beacons.

Directional beam radio transmitters are known in which the direction of transmission is periodically reversed by shifting the phase of the radiated wave in a predetermined time, usually either in time with the code letters e and t or a and u but sometimes in time with other code letters. The result is that a listener who is on one side of a certain plane through the radio beacon will only hear the letter e (or a) whereas a listener on the other side of this plane will only hear the letter t (or n). If the listener is travelling from one side of the radio beacon to the other side of the radio beacon, he will pass a certain position, viz. the said taneously hearing the with equal intensity. formed by shifting the istic of the directional complete each other, so heard.

These radio beacons are usually mentioned as E-T-beacons. The sharpness when determining the direction in which both of the signals are heard with equal intensity is as a rule very good with this kind of radio beacon, and for this reason they are growing to be used more and more for indicating courses on land, on sea or in the air. Although the E-T-beacons are oiiering a plane, where he is simul signal a and the signal 1. As these signals were phase of the characterbeacon, the signals will that a continuous tone is relatively high directional sharpness, they have also certain disadvantages. Especially prominent is the difi'iculty of building phase shifting apparatus with sufiicient precision. Phase shifters used hitherto have as a rule been rather complicated, and it was necessary to make them of rather high precision in order that the desired directional sharpness should really be obtained. These circumstances have essentially counteracted the commercial appearance of E-T-beacons.

In the drawings:

Figs. 1 and 2 show two known types of radio beacons;

Fig. 3 illustrates a transmitter embodying the present invention;

Figs. 4 and 5 are two diagrams showing the operation of the arrangement according to Fig. 3;

Fig. 6 is a schematic diagram showing a modification of the transmitter according to Fig. 3;

Figs. 7 and 8 are schematic two further modifications of the invention; and

Fig. 9 is a diagram to explain the operation of the transmitter of Fig. 8.

In order to better understand the present in vention reference is made to Fig. 1 of the andiagrams showing nexed drawings, which show a known form of E-T-beacon. The transmitter is indicated I. This is connected over a phase shifter 2 to adirectional antenna 3, and over a transformer 4 to a nondirectional antenna 5. The phase shifter contains a set of contacts, w ch are actuated by a code-mechanism 6 in time with the code letters e and t.

The characteristic of the directional frame antenna is, as well known, a curve of substantially the same form as the figure 8, whereas the characteristic of the non-directional antenna is constituted by a circle about the antenna. If now the two loops of the figure 8 should be congruent with the circle diagram of the antenna 5, it is evident that in one phase position one of the loops is in additive relationship to the circle diagram and the other loop is in subtractive relationship, so that a characteristic like a cardioid is obtained which is directed in one of the loop directions. At shifting of the phase the two loops will change function so that the cardioidic characteristic is turned in the other direction.

Systems of this kind were as a rule arranged in such a manner that they work with two contacts for the phase shifting, and it has proven impossible in practice to close and open these contacts simultaneously. Even if they should accidentally be adjusted so that this takes place, the opening spark is sufiicient to destroy the adjustment, and a small difierence of time again appears between the opening of one of the contacts and the closing of the other contact and vice versa. This small difference in time causes an audible click in the receiver, and it has been found that these clicks are highly disturbing and that the precision of the direction finding is thus greatly reduced. The click is heard just at the moment when the listener is trying to compare the sound intensity of successive signals. If no phase shifting click is heard the smallest changes in sound intensity can be readily heard, especially if the code in which the sound intensity is supposed to vary is known in advance. On the other hand the click makes it impossible to observe the change of sound intensity until this has become substantially greater.

In order to obviate the unsharpness in the direction finding caused by these clicks it has been proposed to use capacitive phase displacers or phase shifters, by means of which a continuous phase shifting is obtained. These are, however, rather expensive and require good adjustment. Furthermore, they do not fully eliminate the click.

Finally it has also been proposed to use a bridge arrangement for causing the phase shifting in order to avoid these disadvantages, only one contact in one of the bridge branches being required. The common principle of the system will be obvious from a comparison between the arrangement according to Fig. 1 and the corresponding arrangement shown in Fig. 2. Parts which are common to both of the arrangements are indicated with the same reference numerals.

The contact arrangement 2 has, however, been replaced in the system of Fig. 2, with a bridge circuit containing three equal resistors R and one fourth resistor, containing two series con nected parts R and r. The resistors are so dimensioned, that R+r is R, whereas R is R. When the contact l is opened by the code mechanism 6 current will pass through the circuit 8 to the frame antenna 3, the phase of which is indicated by the arrow 9, whereas when the contact if is closed, current will go through the conductor 8 in the opposite phase, as indicated by the arrow 9'.

In Fig. 2' the bridge circuit has been shown containing resistors. It is, however, obvious, that these resistors cause rather great losses, and in practice the bridge may comprise capacities or inductances or possibly capacities and inductances in a suitable combination. The above stated arrangements are satisfactory at lower frequencies but at higher frequencies, which are used for so-called short wave transmission the bridge arrangement can not be used. It can iot be tuned sufficiently low, and wrongly tuned it causes such a damping of the transmitted alternating current and such a width of the resonance curve, that the sharpness of the directional characteristic is fully lost.

The present invention relates to a method of avoiding these disadvantages, and it consists of using as elements in the bridge circuit the coupling elements represented by the antenna arrangements, that is, the capacitive coupling between two antennas or systems of antennas, the radiating resistance of the antennas, the characteristic of the feed conductor, etcetera. The number of the other coupling elements required is thus brought down to a minimum, and it is possible to use essentially higher frequencies and in spite of that to obtain good tuning.

Fig. 3 shows one embodiment of the invention, in which the capacity of two dipole antennas is used as bridge capacities.

In the arrangement according to Fig. 3 It] is a transmitter for a preferably ultra-short radio wave. This is fed by means of a feeder H to a dipole antenna, containing the bars I2 and i3 and also Hi and E5. The bars l2 and I3 form a pair of bars, whereas the bars M and i form second pair of bars. The distance between the pair of bars lZ-lB and the pair of bars i i-i5 is assumed to be one quarter of a wave length.

There is further provided an arrangement for short-circuiting the dipole antenna between the pair of bars i i and i5, comprising a periodic switch It, which is in this case controlled by a relay H, which is actuated by a code transmitter or a pulse transmitter l8 over the line is. The pulse transmitter 18 opens and closes the switch it to form the e-t or the a-n or possibly some other signal suitable for the purpose. The code, which is used, is of course not essential for the invention.

When the switch it is open, both of the pairs aseaeee of bars i2--l3 and i l-i5 are emitting the signal wave from the transmitter l0. It is to be noted, however, that the signal wave in the direction west, in Fig. 3 indicated W, from the antenna [2-43 will, due to the phase displacement caused in the bridge coupling, be 180 out of phase in relation to the signal wave in the same direction from the antenna I l-45. This is because the signal wave from the transmitter will reach the antenna l2-I3 in a certain phase position, in which it is radiated from this antenna. The same signal wave reaches the antenna M-IE later and is radiated from this antenna. After further a quarter of a period the signal radiated in westward direction from the antenna I i-l5 will have passed the antenna capacity between the antenna I l-45 and the antenna iii-l3, and it has thus reached phase displacement.

If the antennas |2-l3 and i i-l5, respectively, are correctly dimensioned, the action will be that the two signal waves will, due to interference, compensate each other in the westward direction. Ihe antenna will accordingly have the characteristic shown by the curve in Fig. 4, drawn in full lines.

On the other side it is obvious that both of the signals will cooperate in the eastward direction, indicated by E in Figs. 3 and 4.

When the switch I6 is closed, the feeder will have infinite resistance and the transmission from the pair of bars 14-45 will cease. As a consequence hereof the signal wave in the eastward direction is suppressed. The signal wave in the westward direction is, however, transmitted. The characteristic then takes the form shown by the dotted lines in Fig. 4.

The course direction, on which direction finding takes place, is as shown in Fig. 4 as the northsouth-line, which is indicated with N and S.

From the above it is evident, that the distance between the antennas need not necessarily be exactly a quarter of a wave length, but that the same SO-degree phase displacement is obtained when the distance follows the formula L-l-n- /2L, where L is the wave length and n is an arbitrary full number. However, it has been found in practice, that a somewhat better sharpness of direction finding can be obtained if the distance is made somewhat greater than the value calculated according to the above formula, the cardioidic form of the characteristic thereby being somewhat more flattened. In Fig. 5 there is shown with full lines the characteristic at exactly one quarter of a wave length distance between the antennas, and in dotted lines the characteristic when the distance is a few per cent greater than that obtained according to the above stated formula. It is evident from the figure that the curve drawn with dotted lines crosses the north-south-line at an essentially smaller angle, whereby a correspondingly greater sharpness of direction finding is obtained.

The matching of an antenna system of the above described kind is important. Correct matching requires a somewhat greater impedance characteristic of the feeder H than the radiation resistance of each of the dipoles. By experiments it has been proved that the impedance characteristic should preferably be 12% higher than the radiation resistance.

It is not essential that dipoles should be used as antennas. The same directional effect can be obtained with the same simple phase shifting means, utilizing the capacity between two anten Fig. 6 shows as an example a directional system in which standard open antennas are used. The same reference numerals are used as in Fig. 3. The transmitter is indicated at It! and the feeder to the antenna at I I. I

The two antennas are indicated by 20 and 2|, respectively. Preferably the antennas are made as open vertical antennas, and for obtaining a balance in the bridge coupling they should have a height of of a wave length. If the height is less than A; of a wave length, series coils 22 and 23', respectively, should be introduced in the antennas.

A grounded counterpass 24 may be provided in connection with the antennas, in order that the short circuiting by means of the periodic contact l6 shall be more efiective.

The distance between the antennas should also in this case be exactly one quarter of a wave length, provided the normal cardioidic form of characteristic, is desired but if the modified cardioidic form of Fig. 5 is desired, the distance may be increased by a small fraction of a wave length.

When the phase shifting is accomplished, as above described, by closing and opening a contact, there is the risk of forming of sparks and consequently of introducing disturbing clicks in the sound. These clicks, although they are essentially less disturbing than the clicks which were obtained in the earlier systems due to the diiierence in timing between the functions of the difierent switches, nevertheless cause an observable decrease of the sharpness of the direction finding. Particularly the coupling in again of the antenna circuit from its short circuited state when opening the switch I6 has been found to cause very strong opening sparks. These opening sparks are heard as a clicking sound in the receivers and experiences show, that this clicking sound makes it so much more diflicult to listen to the beacon. that the sharpness of direction finding is materially decreased.

According to a further development of the present invention this clicking sound is avoided by making the transmission ineffective during a very short period of time, which coincides with the instant, during which the switch I6 is opening and also, if desired, while the switch is clos ing. Fig. 7 shows a simple arrangement for this purpose.

In Fig. 7 the transmitter is indicated at IE, the antenna feeder at H, the dipole antennas I2, I 3, l4 and 15, the switch 16 and its influencing relay I l. The arrangement of the code transmitter 18 is shown more in detail.

The code transmitter is in this case driven by a motor 25, on the shaft of which a cam disc 25 is mounted. This cam disc carries a cam, which is cut in the same form as the code to be transmitted, for instance with alternating short enlargements 21 (the code letter e) and long depressions 28 (the code letter i). A contact 29 is actuated by the cam disc during its rotation,

so that the contact 29 is closed when passing an e-sector and is open when passing a t-sector. A conductor 30 connects the relay winding I! in series with a suitable source of voltage 3!.

0n the same shaft as the code transmitter is placed a condenser of special construction. The T stator contains two plates 32 and 33, between which ears 34, 35, 36 and 31 projecting from the rotor are adapted to pass. The condenser formed by the plates 32 and 33-and the ears 34 and 3'! is connected across the two conductors means of capacity coupling. The

of the antenna feeder l l by meansof the conductors 38. A damping resistor 39 is connected in the conductor 38. The ears 34, 35, 33 and 3'! of the rotor are so adjusted in relation to the surface of the cam disc, that a capacitive short circuiting of the antenna feeder takes place a very short interval before the phase shifting takes place,.and that this capacitive short circuiting is again extinguished a very short interval of time after the phase shifting has taken place. I

Another arrangement is shown in Fig. 8. Similar reference numerals have been used in Figs. 7 and 8 for corresponding parts.

The transmitter i0 is assumed to contain a control oscillator in one or more control stages, schematicaly indicated 41!], and also an amplifier valve 4|, which is coupled to the oscillator 40 by grid condenser is indicated 42 and the grid resistor 33. An inductance coil 44 may be coupled in series with the grid resistor 43, as shown in the figure. At a suitable place on the grid resistor 43 a tap is provided which is connected to a condenser 45 and by a contact 46 and a resistor 41 to the negative terminal of a source of voltage, schematically shown as a battery 48.

The contact 45 is arranged to be actuated by the cam disc 49, which is mounted on the same shaft as the cam disc 26. Each time the contact 46 is closed, the condenser 45 will be strongly negatively charged, whereby the grid bias is displaced to such a value that the transmitter valve M is blocked. After opening of the contact 46 the condenser 45 is again discharged over that part of the grid resistor G3 which is shunted by the condenser, and the blocking of the valve is extinguished. The variation of the grid bias during transmission of both of the letters E and T is shown in Fig. 9,

Of course the invention is not limited to the specific embodiments described above various changes and modifications may be made without departing from the scope of the invention. Also the invention is not limited to use in connection with short wave transmitters, but may be used with transmitters of any wave length.

What is claimed is:

1. A directional radio beacon system comprising a first tuned antenna, a transmitter feeding the same, a second tuned antenna spaced from said first antenna by about a quarter wave length or an odd multiple thereof, a transmission line joining said antennas to energize the same in relative phase relationship such that the radiated waves oppose in the direction of said first antenna and are in phase in the direction of said second antenna whereby said second antenna acts as a director for such waves to propagate the same unidirectionally, and a single contact means connected to short circuit said transmission line at said second antenna while maintaining said second antenna tuned to thereby interrupt the energization thereof and cause the same to act as a reflector to reverse the direction of wave propagation, whereby said reversal is effected by the opening and closing of a single contact.

2. A directional radio beacon system according to claim 1 in which the antennas comprise tuned dipoles and in which said transmission line is uncrossed between the two antennas, whereby a bridge effect is obtained utilizing the capacitative coupling between corresponding dipole arms of the two antennas, the radiation resistance of said d dipoles, and the impedance of said transmission line.

3. In a directional radio beacon system according to claim 1, means to interrupt the transmission of energy from said transmitter to said antennas at the instants of reversal of wave propagation.

4. In a directional radio beacon system according to claim 1, contact means synchronized with the opening and closing of said first contact to render said transmitter ineffective at the instants of reversal of wave propagation whereby reversing click and distortion are avoided.

5. Radio transmitter according to claim 1 in which the antennas comprise open vertical antennas substantially one quarter of a wave length high.

6. Radio transmitter according to claim 1 in which the antennas comprises open vertical antennas having series coils suited to provide an effective height of substantially one quarter of a wave length.

7. Radio transmitter accordilng to claim 1, characterized in, that the distance between the antennas is a small fraction of one wave length greater than a quarter of a wave length.

8. Radio transmitter according to claim 1 eharacterized in that the impedance characteristic of the transmission line between the antennas is somewhat greater than the radiation resistance of each of the antennas.

9. Radio transmitter according to claim 1, characterized in that the antennas are provided with a counterpass suited to provide an effective grounding of the antennas by means of the periodically acting contact.

CARL-ERIK GRANQVIST. 

